Thank, Wolfgang! Both methods work, but both fail with the following
example:

\pushmacro\unexpanded
\let\unexpanded\normalunexpanded
\input xstring
\popmacro\unexpanded
\starttext
\StrGobbleLeft{bcdef}{4}
\stoptext

The log says:

tex error > tex error on line 6 in file ./tests.tex: ! This can't happen
(prefixed command)


<argument> \unexpanded {

f}

\xs_execfirst #1#2->#1

However, some others work.

El mar., 7 de jul. de 2020 a la(s) 15:20, Wolfgang Schuster (
wolfgang.schuster.lists@gmail.com) escribió:

> Phelype Oleinik schrieb am 07.07.2020 um 20:53:
> > Hi Jairo,
> >
> > ConTeXt's \unexpanded is e-TeX's \protected, so the error boils down to
> > something like: \edef\x{\unexpanded{#}}, which works in Plain but not in
> > ConTeXt.
> >
> > Usually packages that claim to be generic have to cope with that.
> > xstring would need to do (in a ConTeXt-specific loader):
> >      \let\xstring@unexpanded\normalunexpanded
> > and
> >      \let\xstring@unexpanded\unexpanded
> > in Plain and LaTeX loaders, then then use \xstring@unexpanded in the
> > code.  But `xstring` doesn't claim (that I found, at least) to be
> > compatible with ConTeXt, so it's understandable: though if it is
> > compatible with Plain, it wouldn't be hard to adapt to ConTeXt.
> >
> > That said, you can load `xstring` if you do:
> >       \let\unexpanded\normalunexpanded
> >       \input xstring.tex
> >       \let\unexpanded\protected
> > but the internals of the package will probably be comprimised by the
> > misuse of (ConTeXt) \unexpanded.
>
> Bad idea because ConTeXt also changes the meaning of \protected (you can
> use \normalprotected to get the primitve).
>
> A better version is
>
> \pushmacro\unexpanded
> \let\unexpanded\normalunexpanded
> \input xstring
> \popmacro\unexpanded
>
> Wolfgang
>
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